Catalytic cracking today constitutes several refining processes in which heavier hydrocarbons are cracked into lighter useful products. They are fluid catalytic cracking (FCC), hydrocracking, reforming, etc. FCC process is simple and highly flexible as required product slate can be obtained with varying feed properties.
The earliest catalysts used for fixed bed cracking were based on acid reacted clays. The exigencies imposed during the Second World War provided required acceleration to the concept and growth of moving bed catalytic cracking process. This process demanded more rugged catalysts than activated clays. Demand for this was met by more active silica alumina gel based synthetic catalysts, which provided an improved physical stability, and greater selectivity.
Grinding and screening methods were used to produce catalyst particles of required size. Later, spray drying technique produced catalyst particles of required size and with improved attrition resistance suitable for fluid bed reactor systems.
Introduction of silica-magnesia based matrix provided greater selectivity towards the production of middle distillates than silica-alumina based catalysts. One of the most significant developments in the area of cracking catalyst was the introduction of crystalline inorganic synthetic products called “Y zeolites”.
The Y zeolites having discrete pores in the range 6.5 to 13.5° A, higher surface area and higher acidity compared to the amorphous silica-alumina based catalysts generated higher catalytic activity and much more selectivity towards gasoline.
Later on, by employing rare earth exchanged Y zeolites and ultra stable Y zeolites many active and stable catalysts were developed.
Along with the introduction of highly stable cracking components like zeolites in different forms, in the past, there have been gradual improvements in binder system used for binding these crystalline materials into attrition resistant microspheres.
Use of silica sol based binder system in the preparation of zeolite promoted catalysts is cited in U.S. Pat. No. 3,867,308 and alum buffered silica-sol is described in U.S. Pat. No. 3,957,689.
Binding of low soda Y zeolite with gel alumina and polysilicate has been described in U.S. Pat. Nos. 4,333,857 and 4,326,993.
U.S. Pat. No. 4,987,110 refers to the preparation of attrition resistant FCC catalysts using low soda silica sol, REUSY and aluminum chlorohydrol.
Product selectivity of faujasite zeolite (also referred as Y zeolite) based catalysts is restricted to gasoline range molecules, due to the presence of uniform size large pores in the range 6.5° A and 13.5° A. For enhancing C3 to C4 selectivity, for first time ZSM-5 zeolites having pores in the range 5.4 to 5.5° A, were employed along with faujasite zeolites, in a conventional silica-alumina based binder system and process for this is described in U.S. Pat. No. 3,758,403.
U.S. Pat. No. 3,847,793 describes a process for conversion of hydrocarbons with a dual cracking component catalyst comprising ZSM-5 zeolite based catalyst and large pore zeolite based catalyst.
U.S. Pat. No. 6,258,257 refers to a process for producing polypropylene from C3 olefins by a two-stage fluid catalytic cracking process having two types of catalysts made from zeolites of large pore and medium pore.
U.S. Pat. No.6,137,022 discloses a process of making an olefin product from an oxygenated feedstock by contacting the feedstock in a reaction zone containing 15 volume percent or less of a catalyst, preferably a catalyst comprising a silica-alumina-phosphate molecular sieve.
Another method of adding ZSM-5 to a moving bed catalytic cracking unit is disclosed in the published European Application No. EP 0167325A3. The make-up catalyst may contain 2 or 3 times the amount of ZSM-5 sought for the equilibrium catalyst.
U.S. Pat. No. 6,156,947 refers to a process for jointly producing butene-1 and ether in a catalytic distillation column, which comprises an upper catalytic zone for etherification and a lower catalytic zone for isomerization of C3 to C4 olefins and conversion of butadiene.
U.S. Pat. No. 5,997,728 refers to a process for catalytically cracking of a heavy feed in a FCC unit, with large amounts of shape selective cracking additive. The catalyst inventory preferably contains at least 10 wt % additive, of 12-40% ZSM-5 on an amorphous support, equivalent to more than 3.0 wt % ZSM-5 crystal circulating with equilibrium catalyst. This process yields large amount of light olefins, without excessive production of aromatics, or loss of gasoline yield.
U.S. Pat. No. 4,309,280 describes a process for maximizing of LPG by adding very small amounts of powdered, neat ZSM-5 catalyst, characterized by a particle size below 5 microns to the FCC catalyst inventory.
U.S. Pat. No. 5,190,902 refers to a process for the preparation of attrition resistant binder particles by spray drying of clay phosphate slurry with adjusted pH. At extreme pH conditions (pH above 12 and below 2), aluminum in clay is converted to Al++−ions and on calcination in presence of phosphate ions, forms aluminum phosphate binder. This reaction condition is exploited in developing attrition resistant microspheres. It may be noted from the different examples that, clay-phosphate based binder is particularly suitable for zeolites with higher silica to alumina ratio and not suitable for binding low SiO2 to Al2O3 ratio (ratio below 6) zeolites. This is due to the fact that, at lower pH conditions alumina of zeolite is also likely to get ionized and phosphate will not differentiate between alumina of clay and alumina of zeolite and react with both. This leads to the removal of alumina of zeolite framework, there by substantially reducing catalytic activity of the zeolite.
U.S. Pat. No. 5,286,369 describes a phosphate based binder composition suitable for binding high silica zeolites. Here, reaction between aluminum nitrate and phosphoric acid forms aluminum phosphate binder. However, during the reaction, along with the formation of aluminum phosphate binder, nitric acid is also formed as a by-product as per following reaction.Al(NO3)3+H3PO4→AlPO4+3HNO3
Presence of nitric acid is detrimental to the stability of Y zeolites due to dealumination.
Recent work on ZSM-5 additive has been directed at stabilizing the zeolite and also making the additive more attrition resistant. Phosphorus stabilized ZSM-5 additive is believed to retain activity for a longer time. ZSM-5 zeolite based catalysts in absence of phosphate have tendency to get dealuminated under hydrothermal conditions.
Conventional silica-alumina binders used for binding Y type zeolites cannot be used for binding ZSM-5 zeolites, as they do not contain required phosphate for stabilizing these zeolites. Clay-phosphate based binders used for binding ZSM-5 zeolites cannot be used for binding Y zeolites having lower SiO2 to Al2O3 ratio, due to severe dealumination which results in loss of catalytic activity.
While Y zeolite based FCC catalysts with conventional silica-alumina binder selectively crack heavy feed molecules into gasoline range molecules, ZSM-5 zeolite based catalysts bonded with clay phosphate binder selectively produce higher amount of C3 to C4 olefins by selective cracking of paraffins and alkyl aromatics present in the feed.
Differences in cracking pattern with two catalysts made with ZSM-5 and Y zeolite is attributed to the size and architecture of the pores present in these two zeolites. These two zeolites cannot be bonded with a common binder. Present practice of employing ZSM-5 zeolites for conventional FCC reaction is through making separate additive catalyst with a phosphate based binder and these catalysts are called as ZSM-5 additives. These additives are used in the range 1 to 6 wt % level of total catalyst present in inventory. Further increase in ZSM-5 additive catalyst in the inventory reduces the catalytic conversion, due to the dilution effect. This limitation in conversion on using additives in place of FCC catalysts, is due to the fact that, only medium pores in the range 5.4 to 5.5° A are present in ZSM-5 zeolites. These pores will not permit the entry of bulky hydrocarbon molecules for cracking. This limitation is known as “reactant selectivity” and is explained in an issue of “Zeolites” 4, 203 (1984). Further, distribution of ZSM-5 zeolite particles is restricted only on additive catalyst particles and hence effective use of this zeolite is limited.
Hence, there is a need to develop a process for a cracking catalyst, which can simultaneously bind both large pore Y zeolite with low SiO2 to Al2O3 ratio and medium pore, high silica ZSM-5 zeolite. Formulation of single particle cracking catalyst with two types of zeolites will address the limitations encountered with the existing catalyst formulations.